1. Field
The present invention relates to a cathode for a lithium secondary battery and a lithium secondary battery comprising the same. More particularly, the present invention relates to a cathode for a lithium ion polymer battery that uses olivine-type lithium iron phosphate as a cathode active material and that has high energy density, and a lithium secondary battery comprising the same.
2. Description of Related Art
Recently, there is an increasing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers, and even electric cars, the demand for high energy density of batteries used as power sources of electronic equipment has been increasing. Lithium secondary batteries are given attention as the most favorable battery capable of meeting the demand, and currently studies are being actively made on lithium secondary batteries.
Lithium secondary batteries developed in the early 1990's are made up of an anode of a carbon-based material capable of intercalating and deintercalating lithium ions, a cathode of lithium containing oxide, and a non-aqueous electrolyte containing a proper amount of lithium salts dissolved in a mixed organic solvent.
As a cathode active material of a lithium secondary battery, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or lithium composite metal oxide (Li(Ni—Co—Al)O2, Li(Ni—Co—Mn)O2) is used. Among them, lithium cobalt oxide has a layered crystal structure of O3, which makes it easy to intercalate and deintercalate lithium ions, and thus, is currently used in a majority of lithium secondary batteries.
However, studies have been made to develop new cathode active materials since cobalt, a raw material of lithium cobalt oxide is a costly heavy metal that is not environmental friendly. As alternative cathode active materials, spinel-type lithium manganese oxide (LiMn2O4) and an olivine-type lithium iron phosphate compound (LiFePO4) that are low in cost and have high stability have been suggested.
Of them, olivine-type lithium iron phosphate has a very stable structure. Also, olivine-type lithium iron phosphate is excellent in thermal stability because phosphate-based materials are used as a flame retardant. Accordingly, olivine-type lithium iron phosphate is qualified as a cathode active material capable of meeting the high stability demand of lithium secondary batteries that has been increasingly emphasized recently.
However, when the solid content of olivine-type lithium iron phosphate is 80 wt. % or more, a cathode active material slurry has an excessively high viscosity that is awkward to convey through a pipe and to coat a current collect, which makes it difficult to manufacture a cathode. Conversely, when the solid content of olivine-type lithium iron phosphate is less than 80 wt. %, it will be a chief obstacle in meeting the rising demand for high energy density batteries.